**5. Complex Coating Architectures**

Coatings used in industrial applications are seldom homogeneous monolayers, but often comprise several coating layers or complex architectures. This allows several functional requirements to be met through coating layers serving different functions, for example, good adhesion to the substrate material through a bonding layer, mechanical stability through a main layer, and tribological properties through a top layer. Furthermore, in nanocomposite or multilayered designs, the additional interfaces created can help to enhance performance, for example, by providing increased fracture resistance, beyond the properties of the individual layers. A multitude of approaches have been demonstrated for AlCr*X*-based coatings.

A schematic drawing of the main coating architectures is shown in Figure 22. It must be noted that the bond coat which is often applied essentially produces a two-layer coating that has not been considered explicitly in this overview.

**Figure 22.** Main types of coating architectures.

It should be mentioned that in practical coating designs, different architecture types are often mixed in one and the same coating, e.g., gradient coatings are predominantly realised more or less stepwise. The nanocomposite coating shown might be a part of the multilayer coating or part of the gradient coating. A primary example of a nanocomposite structure is the system (AlCrSi)N/a-SiN*x*, as discussed in Section 4.4.
